Corey Linsley didn't know anything was wrong until he showed up for work Sunday morning.

The Green Bay Packers rookie center was like so many others who figured JC Tretter would be fine coming out of Friday night's 31-21 preseason win over Oakland at Lambeau Field.

What many didn't notice was Tretter grimacing in pain after the team's first offensive series. He had his left knee examined briefly by team doctor Patrick McKenzie before returning to play all 45 snaps with the offensive starters.

It wasn't until Sunday that Linsley was told he would be taking his first reps with the first-team offense. A few hours later, Packers coach Mike McCarthy confirmed Tretter sustained a "significant knee injury" against the Raiders and will be sidelined for "multiple weeks."

There was concern the prognosis could be worse, but ESPN Wisconsin reported Tretter didn't suffer any ligament damage and shouldn't be sidelined more than six weeks.

Meanwhile, the offense turns to Linsley, a fifth-round pick in May's draft who has a little less than two weeks to prepare for the Packers' regular-season opener against Seattle inside deafening CenturyLink Field.

"The urgency level is just through the roof," Linsley said. "I've been working hard, but it's a different animal out there with the ones. I've got to fill in at the highest level. There's no room for nonsense anymore and ridiculous mistakes."

After allowing veteran Evan Dietrich-Smith to depart for Tampa Bay in free agency, the Packers spent the summer grooming Tretter to be the starter and long-term answer to a position in a constant state of flux.

A fourth-round pick out of Cornell last year, Tretter was off to a promising start to camp despite not playing in a game his rookie season because of a broken ankle he suffered on the first day of organized activities.

Packers center JC Tretter holds his knee on the goal line after he was injured on a running play in the first quarter Friday against the Raiders at Lambeau Field.(Photo: Jim Matthews/Press-Gazette Media)

The Packers' recent history with injuries have conditioned them for this scenario, especially on the offensive line.

They inserted undrafted rookie Don Barclay in at right tackle midway through the 2012 season when Bryan Bulaga suffered a season-ending hip injury. Last August, fourth-round pick David Bakhtiari was thrust into the starting left tackle role when Bulaga suffered a torn ACL.

The center position has been a revolving door for different reasons. Dietrich-Smith's departure sent quarterback Aaron Rodgers onto his fourth starting center in as many years.

That still holds true. It's just not the center everyone was anticipating.

"This will be my millionth center, so it's not anything new to us," left guard Josh Sitton said. "I wouldn't say it's difficult; it's just a process. I told JC this morning, 'Right when I got real comfortable with you, now you're gone.' It sucks, it's unfortunate, but we're used to working with a bunch of guys, so it's something you've got to roll with."

Linsley's lessons started Sunday between Sitton and right guard T.J. Lang. There were a few moments in practice when the veterans had to point something out to the rookie reading the defense, but they felt it went fine overall.

Physically, Linsley plays like a man capable of bench-pressing 500 pounds, one of the attributes the Packers most admired about him during the draft process. His natural strength was reflected in his 8-1 record in the one-on-one drills earlier in camp.

On the field, Pro Football Focus saw Linsley concede only one pressure in 59 pass-blocking snaps this preseason, but he still sees plenty of room for improvement when he looks at the film.

That's where having Sitton and Lang to lean on could make a difference.

"It's definitely the mental side of the game that I've been slacking on and that I need to improve on," Linsley said. "It's just the subtleties, the outside zone step as opposed to the inside zone step, the differences between the aiming point are very subtle, but they make a difference. That's what I've got to work on."

The center is an important cog in the Packers' offense, especially if McCarthy is determined to keep running the no-huddle with the same fluidity they have in the preseason.

Linsley has worked extensively in that capacity with backup candidates Matt Flynn and Scott Tolzien, but there's a chance he might not take an in-game snap with Rodgers until the opener against Seattle on Sept. 4.

The Packers rested Rodgers in last year's preseason finale against Kansas City and could do the same this Thursday against the Chiefs. Rodgers sounded like a quarterback who had played his last snap when he told reporters he felt the offense was ready after Saturday's win over the Raiders.

The Packers must now decide if Tretter's absence changes that assessment.

"The timing kind of sucks just because it's so late in training camp that we haven't had a whole lot of reps together," Lang said. "This week, especially the fourth preseason game, I think history shows the starters won't play a whole lot. It's going to be extra stuff we're going to have to do on the practice field and meeting rooms, making sure we're trying to build chemistry in other ways outside of actually playing games together because we won't have that opportunity for probably another week and a half."

McCarthy said he didn't have enough information on Tretter's injury to determine if he could be the team's designated player to return from injured reserve. If the 5-to-6-week time line is accurate, that wouldn't be necessary since a player placed on temporary injured reserve is required to miss eight weeks.

For the moment, the job belongs to Linsley, though they have options if the rookie proves not to be ready. Either Sitton or Lang have snapped before and could slide to center with second-year reserve Lane Taylor filling in at guard.

At the moment, the offensive line feels confident they'll adapt just like have the past few seasons.

"It can be tough. We ask a lot out of our centers," Lang said. "But I think it'll be a lot smoother than everybody is anticipating."